Depends, if you had max load on the electrical system then dropping to only 12V like that...well...I don't know if it's 'normal' but that's what mine does under max load. If the engine is just running with no electrical load on it then it shouldn't be doing that. How did you 'test' it?
Stereo has been in the car for 3 years.....turned down to 0 to hear the engine. only 4 speakers - no amp